The invention relates to a hydraulically operated rock drill of the percussive type. The rock drill represents an improvement in a well known and crowded art, and features a combination of various means, some known in the art, to provide a rock drill of exceptional operational effectiveness.
Various attempts have been made in the rock drill art to provide what is frequently referred to as "optimum pulse width" with referring to the shape of the stress wave in the drill steel, as displayed on an oscilloscope. Optimum pulse width is three to eight times greater than that produced by a conventional piston hammer striking a conventional drill steel, and it varies with the hardness or stiffness of the rock being drilled. Reference may be had to U.S. Pat. No. 3,796,271, and the patents referred to therein, for a discussion of the problem and suggested solutions thereto. The present invention utilizes a long, heavy piston which has a short stroke and low impact velocity to obtain favorable pulse duration. While the ratio of piston length to drill housing length is in the order of 60%, the piston diameter is substantial, thus providing a heavy piston which minimizes overall tool length.
A further feature of the rock drill of the invention is the use of only two sliding seal diameters-contemporary rock drills often employ three diameters which require superprecision manufacture for proper assemblage- the two-diameter arrangement permitting simplest possible valve and cylinder configuration leading to reduction in manufacturing and maintenance costs. A semi-mechanical cycle valve is employed to provide the unusually short stroke that is required when full cross-section of piston is exposed to hydraulic pressure for power-stroke acceleration.
The rock drill of the invention, while utilizing a reservoir of high pressure oil which also serves as the drill housing, as in U.S. Pat. No. 3,892,279, represents an improvement thereover by use of reservoir oil for lubrication and cooling of front head assembly sliding surfaces, and also by full enclosure of anvil rotation means driven by a rear mounted motor, the latter arrangement being less cumbersome and of reduced costs as compared with front head rotation motor constructions.
Still another feature of the invention relates to a floating seal cartridge which provides rotation of the piston with the anvil, thus minimizing rotative scrubbing at impact of the piston upon the anvil.
Other advantages and features of the rock drill of the invention will become apparent as the description of the invention follows in greater detail.